U.S. patent number 4,817,053 [Application Number 07/071,309] was granted by the patent office on 1989-03-28 for apparatus for storing and retrieving information using an electron beam.
This patent grant is currently assigned to Hitachi, Ltd.. Invention is credited to Shinkichi Horigome, Tadashi Ikeda, Yasushi Miyauchi, Tetsuya Nishida, Norio Ohta, Kazuo Shigematsu, Ryo Suzuki, Motoyasu Terao.
United States Patent |
4,817,053 |
Ikeda , et al. |
March 28, 1989 |
Apparatus for storing and retrieving information using an electron
beam
Abstract
An electron beam memory system in which a phase transition type
recording film is used as an information recording medium, and for
recording information, a focused electron beam is selectively
projected on desired positions of the recording film so as to
locally heat the recording film and cause phase transition, while
for retrieving information, an electron beam having energy at a
degree not causing the phase transition is projected so as to
utilize the fact that reflection or diffraction of the primary
electron beam projected at the retrieval differs between the
recorded regions and unrecorded regions.
Inventors: |
Ikeda; Tadashi (Kanagawa,
JP), Shigematsu; Kazuo (Saitana, JP),
Miyauchi; Yasushi (Tokyo, JP), Terao; Motoyasu
(Tokyo, JP), Nishida; Tetsuya (Tokyo, JP),
Horigome; Shinkichi (Tokyo, JP), Ohta; Norio
(Sayama, JP), Suzuki; Ryo (Tokyo, JP) |
Assignee: |
Hitachi, Ltd. (Tokyo,
JP)
|
Family
ID: |
15742893 |
Appl.
No.: |
07/071,309 |
Filed: |
July 9, 1987 |
Foreign Application Priority Data
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Jul 11, 1986 [JP] |
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61-161838 |
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Current U.S.
Class: |
365/113;
250/214LA; 365/118; 365/128; 369/13.01; G9B/9.025 |
Current CPC
Class: |
G11B
9/10 (20130101) |
Current International
Class: |
G11B
9/10 (20060101); G11B 9/00 (20060101); G11C
013/00 () |
Field of
Search: |
;365/118,128,217,113,237
;369/13 ;346/158 ;250/213R ;313/507 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0139474 |
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May 1984 |
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EP |
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0158804 |
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Oct 1985 |
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EP |
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1486271 |
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Sep 1977 |
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GB |
|
Other References
IBM Tech. Discl. Bull., vol. 16, No. 3, Aug. 73, p. 869
"Electron-Beam Addressable Memory By Solid-State Transformation" by
Chaudhari et al. .
IBM Tech. Discl. Bull., vol. 16, No. 1, Jun. 1973, pp. 108-109,
"Structures for Reversible SmS Optical Systems" by von
Gutfeld..
|
Primary Examiner: Hecker; Stuart N.
Assistant Examiner: Garcia; Alfonso
Attorney, Agent or Firm: Antonelli, Terry & Wands
Claims
We claim:
1. An apparatus for storing and retrieving information using an
electron beam, comprising:
a solid phase transition recording medium which is disposed on a
disc-shaped substrate, solid phase transition of said recording
medium occurring in accordance with an amount of irradiation with
an electron beam, said solid phase transition of said recording
medium occurring between a first solid phase and a second solid
phase;
a recording electron gun which emits said electron beam, and which
irradiates said recording medium with said electron beam at a level
corresponding to information and causes the solid phase transition
of said recording medium so as to cause the information to be
recorded into said recording medium;
a retrieval electron gun which emits an incident electron beam of
primary electrons so as to irradiate said recording medium with the
incident electron beam, the incident electron beam being at a level
not causing the solid phase transition in said recording medium,
said retrieval electron gun being adapted to permit information to
be read out of said recording medium;
means for reading out the information recorded in said recording
medium, in such a way that a state of the solid phase transition of
said recording medium is detected on the basis of an amount of
reflected primary electrons of said incident electron beam from a
surface of said recording medium;
means for rotating said disc-shaped substrate; and
means for moving said recording electron gun, the retrieval
electron gun and the means for reading out the information.
2. An apparatus for storing and retrieving information using an
electron beam according to claim 1, wherein a protective film for
avoiding deformation of the recording medium upon the solid phase
transition is provided on said recording medium, the detection of
the state of the phase transition being performed on the basis of
the amount of reflected primary electrons from a surface of the
recording medium through the protective film.
3. An apparatus for storing and retrieving information using an
electron beam according to claim 2, wherein said protective film is
made or SiO.sub.2.
4. An apparatus for storing and retrieving information using an
electron beam according to claim 3, wherein said protective film
has a thickness of 30 nm to 200 nm inclusive.
5. An apparatus for storing and retrieving information using an
electron beam according to claim 1, wherein said recording electron
gun is spaced about 2 mm from the surface of said recording medium,
and said retrieval electron gun as well as the means for reading
out the information is spaced about 2 .mu.m from the recording
medium surface.
6. An apparatus for storing and retrieving information using an
electron beam according to claim 1, wherein said means for reading
out the information is an electron multiplier which is arranged
around said retrieval electron gun.
7. An apparatus for storing and retrieving information using an
electron beam according to claim 1, wherein said means for reading
out the information is a channel plate which is arranged around
said retrieval electron gun.
8. An electron beam memory system according to claim 1, wherein the
rotation means rotates at a speed of 3000-5000 r. p. m.
9. An apparatus for storing and retrieving information using an
electron beam according to claim 1, wherein the first solid phase
is an amorphous phase and the second solid phase is a crystalline
phase.
10. An apparatus for storing and retrieving information using an
electron beam according to claim 1, wherein said recording electron
gun emits an electron beam of sufficient energy so as to locally
heat the recording medium and cause the solid phase transition
between the first solid phase and the second solid phase.
11. An apparatus for storing and retrieving information using an
electron beam according to claim 1, wherein the recording electron
gun emits an electron beam having a diameter of at most 1.0
.mu.m.
12. An apparatus for storing and retrieving information using an
electron beam according to claim 11, wherein the recording electron
gun emits an electron beam having a diameter of at most 0.5
.mu.m.
13. An apparatus for storing and retrieving information using an
electron beam, comprising:
a solid phase transition recording medium which is disposed on a
disc-shaped substrate, the solid phase transition of said recording
medium corresponding to the information;
means for recording information in said recording medium, the
information recording means emitting an electron beam which
irradiates said recording medium at a level corresponding to the
information and which causes the solid phase transition of said
recording medium;
means for reading out the information recorded in said recording
medium, the means for reading out the information including a means
for emitting an incident electron beam and irradiating said
recording medium with said incident electron beam, said incident
electron beam being at an energy level that does not cause the
solid phase transition in the recording medium, and means for
detecting a state of the solid phase transition of said recording
medium on the axis of an amount of reflected primary electrons of
said incident electron beam from said recording medium;
means for rotating disc-shaped substrate; and
means for moving said means for recording information, said means
for emitting an incident electron beam, and said means for
detecting.
14. An apparatus for storing and retrieving information using an
electron beam according to claim 13, wherein a protective film for
avoiding deformation of said recording medium upon the phase
transition is provided on said recording medium, the means for
detecting acting to detect the amount of reflected primary
electrons from the surface of the recording medium through the
protective film.
15. An apparatus for storing and retrieving information using an
electron beam according to claim 14, wherein said protective film
is made of SiO.sub.2.
16. An apparatus for storing and retrieving information using an
electron beam according to claim 15, wherein said protective film
has a thickness of 30 nm to 200 nm inclusive.
17. An apparatus for storing and retrieving information using an
electron beam according to claim 13, wherein said means for
emitting an incident electron beam and the means for detecting are
each spaced about 2 .mu.m from the surface of said recording
medium.
18. An apparatus for storing and retrieving information using an
electron beam according to claim 13, wherein said means for
detecting is an electron multiplier which is arranged around said
means for emitting an incident electron beam.
19. An apparatus for storing and retrieving information using an
electron beam according to claim 13, wherein said means for
detecting is a channel plate which is arranged around said means
for emitting an incident electron beam.
20. An apparatus for storing and retrieving information using an
electron beam according to claim 13, wherein the rotation means is
rotated at a speed of 3000-5000 r. p. m.
21. An electron beam retrieval system according to claim 13,
wherein the phase transition of said recording medium occurs
between an amorphous phase and a crystalline phase.
22. An apparatus for storing and retrieving information using an
electron beam according to claim 13 wherein said information
recording means emits an electron beam of sufficient energy to
locally heat the recording medium and cause the solid phase
transition.
23. An apparatus for storing and retrieving information using an
electron beam, comprising a solid phase transition recording film
used as an information recording medium; means for providing a
focused electron beam, wherein, for recording information, the
focused electron beam is selectively projected on desired positions
of the recording film so as to locally heat the recording film and
cause solid phase transition, to provide recorded regions and
unrecorded regions; means for providing an incident electron beam
for retrieving information, wherein an electron beam having energy
at a degree not causing the solid phase transition is projected;
and detecting means for retrieving the information by detecting a
difference in the reflection of primary electrons of the incident
electron beam projected at the retrieval between the recorded
regions and unrecorded regions.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a system for recording and
retrieving information. More particularly, it relates to an
electron beam memory system of the phase transition type wherein
the phase transition of a recording medium is effected with an
electron beam and wherein the state of the phase transition is
detected with an electron beam.
2. Description of the Related Art
As systems for recording and retrieving digital information, there
have been known a magnetic disc system, an optical disc system,
etc. Any of these systems performs recording and retrieval in such
a way that a physical or chemical change is caused in a recording
medium by the use of a magnetic flux signal or an optical signal,
and that the presence and absence of the physical or chemical
change correspond to digital information items "1" and "0."
A system utilizing the phase transition of a recording medium as
the physical or chemical change thereof is disclosed in U. S. Pat.
No. 3,530,441. This system is such that atoms in the recording
medium are moved by heating with a laser beam, so as to generate
local phase transition states in the recording medium. However, in
case of recording information by the use of light in this manner,
the smallest light beam spot which can be converged is of
approximately 1 .mu.m in diameter because of the limit of the
wavelength of the light, and the limit of the recording density of
the system has been 10.sup.8 bits/cm.sup.2.
Meanwhile, an electron beam recording and retrieving method wherein
information is recorded and retrieved by employing an electron beam
instead of the laser beam is disclosed in the official gazette of
Japanese Patent Application Laid-open No. 62845/1983. This method
is such that a recording medium which comprises a substrate, a
chalcogenide thin film (Se-Ge-based thin film) disposed on the
substrate, and an Ag thin film formed on the chalcogenide thin
film, is irradiated with the electron beam so as to diffuse Ag into
the chalcogenide thin film on the basis of the electron beam
exposure, thereby to record information, while the difference
between the secondary electron emissivities of exposed parts and
unexposed parts is sensed, thereby to read the information. Since,
however, the Ag thin film disposed on the chalcogenide thin film
has the disadvantage of liability to oxidation in the atmospheric
air, the method involves the problem that the retrieval efficiency
of information lowers due to the oxidation of the surface of the Ag
thin film. Another problem is that, since information is recorded
by exploiting the diffusion of Ag into the chalcogenide thin film
as based on the electron beam exposure, it cannot be rewritten.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an electron beam
memory system which can record high-density information at a
recording density of or above 10.sup.8 bits/cm.sup.2.
Another object of the present invention is to provide an electron
beam memory system which is non-volatile and rewritable.
Still another object of the present invention is to greatly
increase the retrieval efficiency of an electron beam memory
system.
According to the present invention, the diameter of an electron
beam is set at or below 1 .mu.m, more preferably at or below 0.5
.mu.m, and a phase transition type recording film is locally
irradiated with the electron beam which is pulsed or whose
intensity is changed, whereby the thin film is locally heated and
subjected to phase transition so as to write or erase information.
Phase transition regions of submicron diameter written by this
operation can be read out at high speed and at high efficiency.
Further, according to the present invention, information is read
out by directly sensing or detecting the reflection or diffraction
of primary electrons, not secondary electrons emitted by the
electron beam irradiation. The reason therefor is as follows: The
energy of the secondary electrons emitted from the phase transition
film amounts to at most several % of that of the primary electrons
in many cases though this depends also upon the material of the
film. Especially a primary electron beam for retrieval must be
weakened to the extent of affording no damage to the recording
medium, so that the energy of the secondary electrons for detection
becomes still feebler. It is therefore very difficult to detect a
difference based on the phase transition.
Besides, the surface of the phase transition type recording film
being the recording medium is sometimes formed with a protective
film for avoiding the deformation of the medium attendant upon the
phase transition. In this case, the secondary electrons from the
recording medium scatter also in the protective film portion.
Especially in the case of disposing the protective film, therefore,
it is next to impossible to detect the phase transition regions
beyond the protective film.
For these reasons, with note taken of back scattering for the
incident primary electron beam, the inventors studied the read-out
of information from the phase transition material beyond the
protective film. As a result, it has been found out that the
amounts of detection of reflected electrons are clearly different
between an amorphous phase where information is written and a
crystalline phase where information is not written, and that the
detection amounts of the reflected electrons conspicuously change
particularly at the boundary part between the phases. Thus,
information read-out of high signal-to-noise ratio has been
realized by utilizing the property that, unlike those of the
secondary electrons based on incident electrons, the detection
amounts of the reflected electrons (or absorbed electrons) are
greatly different between the information written part and the
unwritten part. Since the read-out efficiency has increased, it has
become possible to sharply shorten a period of time required for
the detection and to remarkably raise a read-out speed. That is,
owing to the detection of the reflected electrons, the read-out of
high efficiency and high speed has been permitted, and further, the
detection through the protective film of SiO.sub.2 or the like has
been permitted, so that the aforementioned objects can be
accomplished.
The above-stated phase transition information retrieval method
which exploits the reflection or diffraction of the irradiating
electron beam operates so as to increase a detection output much
more than in the case of exploiting the secondary electron beam. As
a result, the signal-to-noise ratio (S/N ratio) is greatly improved
in a memory which adopts the retrieval method. Since the detection
sensitivity is sharply enhanced, the period of time required for
reading one information item is shortened much. Therefore, even
when the rotational speed of a disc is increased to read out
information at high speed, no malfunction occurs. Besides, a
protective film is usually required for a phase transition type
recording film in order to avoid the deformation thereof in the
operation of rewriting recording. With the secondary electron
read-out, the detection beyond the protective film has been
difficult because of the low energy of the emitted electrons. In
contrast, with the retrieval method of the invention, the reflected
electrons the energy level of which is as high as that of the
incident electrons, and hence, the read-out beyond the protective
film is possible, so that the practicability of this method is
high. By the way, when the protective film on the side of electron
beam incidence is too thin, it does not have the effect of checking
the deformation, whereas when it is too thick, the S/N ratio of
read-out becomes low. The appropriate thickness of the protective
film depends closely upon the material thereof. For example, in
case of employing SiO.sub.2, the thickness of the protective film
should preferably be 30 nm to 200 nm inclusive.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a view schematically showing an electron beam memory
system which is an embodiment of the present invention.
FIG. 2 is an expanded view of a retrieval electron gun in FIG.
1.
FIG. 3 is a diagram showing the state of reflected electron
currents.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Now, one embodiment of the present invention will be described with
reference to FIGS. 1 and 2.
FIG. 1 is a schematic view of the electron beam memory system of
the present invention, while FIG. 2 is an expanded view of a
retrieval electron gun 5. A recording material 2 formed on a
disc-shaped substrate 1 is made of In-Se-Tl, and it is a phase
transition material film having a thickness of 120 nm. This film 2
is sandwiched between SiO.sub.2 protective films 10 which are 150
nm thick on the substrate side and 80 nm thick on the front surface
side of the film (the protective film on the substrate side is not
shown). The disc substrate 1 is fixed to a cylinder 3, and can
rotate at 1000-4000 r. p. m. Electron guns 4 and 5 which are
recording and retrieval heads, respectively, are installed over the
disc substrate 1. Of course, a single electron gun serving for both
recording and retrieval is sometimes installed, but the case of
employing the dedicated heads will be explained here. The recording
electron gun 4 is about 2 mm spaced from the surface of the disc 1,
and is adapted to generate an electron current of 1 .mu.A under an
acceleration voltage of 3 kV. The diameter of an electron beam from
this gun 4 is focused to 0.3 .mu.m.
The recording film 2 is rendered crystalline before the irradiation
thereof with the electron beam. However, it has the property that,
when the temperature of an irradiated part is raised up to
approximately 700.degree. C. and the electron current is rapidly
decreased, the irradiated part is quenched to undergo phase
transition into an amorphous phase. In addition, although the
In-Se-Tl film is covered with the SiO.sub.2 being 80 nm thick, the
electron current satisfactorily reaches the front surface of the
recording film 2. The recording film heated by the electron current
reaching the film surface turns into the desired amorphous phase,
whereby information is written. On the other hand, the erasure of
information is achieved in such a way that the recording film 2 in
which the information has been written has its temperature raised
to 500.degree. C. so as to be restored into the crystalline state.
The temperature rise on this occasion is carried out by
continuously projecting an electron beam. "Overwrite" (in which
rewrite is realized merely by recording without an erasing
operation) is also possible when the recording film is irradiated
with an electron current which is intensity-modulated between an
electron current intensity bringing the surface temperature of the
recording film 2 to approximately 500.degree. C. and an electron
current intensity bringing it to approximately 700.degree. C.
The characterizing feature of this system consists in the retrieval
of recorded information. The head dedicated to the retrieval 5,
over the disc substrate 1, is lighter in weight than the recording
head 4, and is easier of moving. This retrieval electron gun 5 is
used in the state in which it is brought close to the disc
substrate up to a spacing of approximately 2 .mu.m. The retrieval
electron gun 5 is adapted to emit an electron beam having an
electron current of 40 nA and a beam diameter of 0.2 .mu.m under an
acceleration voltage of 500V. The reflected electrons 7 of the
electron current 6 emitted from the retrieval electron gun 5 are
detected by a channel plate 8 (electron multiplier) which is
disposed around this electron gun 5. Here, the reflected electrons
whose detection sensitivity differs greatly in accordance with the
presence or absence of the phase transition unlike secondary
electrons are detected, so that the information can be detected
through the protective film 10, and the signal-to-noise ratio of
the detection is favorable. Moreover, owing to the retrieval head 5
which is sufficiently lighter than the recording head 4, the
information can be read out at a high speed of 3000-5000 r. p.
m.
FIG. 3 is a diagram showing the state of reflected electron
currents. As apparent from the diagram, the amounts of the
reflected electrons differ depending upon the crystalline phase (at
21a, 21b and 21c) and the amorphous phase (at 22a and 22b), but the
reflection amount is often larger in the vicinity of the boundary
between the phases, than in any of the phases. There is also a case
where the reflection amount becomes smaller contrariwise. Anyway,
the presence or absence of the phase transition can be readily
detected near the boundary of the phases. Accordingly, the present
invention adopts a recording method in which the position of the
phase boundary corresponds to information, thereby to realize a
recording density which is about twice as high as the density of a
conventional method (a method in which the central position of a
phase corresponds to information). Incidentally, those parts of the
disc substrate, the electron gun heads and the detector in the
present embodiment through which the electrons pass are held in a
vacuum vessel 9.
Although the erasable electron beam memory has been described in
detail here, it is needless to say that the present invention is
also applicable to a read-only memory (ROM), a write-once memory
and an add-on memory without any modification.
In the above description, the phase transition has been caused
between the crystalline phase and the amorphous phase. However, it
may well be caused between a first crystalline phase and a second
crystalline phase or between a first amorphous phase and a second
amorphous phase.
* * * * *